This application proposes to use the Siamese fighting fish as a model for studying neural mechanisms underlying aggression. This fish is one of the most aggressive animals known. It engages in agonistic display and fights with conspecifics under almost any circumstances. The agonistic display is characterized by a repertoire of stereotyped postures, including tonic gill cover extension, fin erection, and color intensification; both neural and neuroendocrine controls are implicated in this display. Behavioral studies have demonstrated that this display is a reliable index of the aggressive state of the animal. However, agonistic display habituates with prolonged exposure to a conspecific, suggesting that there are intrinsic mechanisms that constrain aggressiveness. Understanding these mechanisms may provide insights into the control of aggression. Gill cover extension is mediated by a single muscle, the dilator operculi (DO), which controls both the rhythmic respiratory movement and the tonic agonistic display of the operculum. Tonic firing of DO motoneurons is thus a physiological manifestation of the aggressive state, and will be used in this study as an entry into understanding the central control of aggression. Three sets of experiments are proposed: (1) Detailed behavioral studies will be performed by analyzing video records of the display. Electromyographic recordings of the DO will also be made. Efforts will be focused on habituation of agonistic display. These studies will provide valuable information on the nature of the motor program controlling this behavior. (2) Electrophysiological recordings will be performed to define properties of DO motoneurons. Of particular interest are the types of inputs that elicit tonic firing in some of these cells. This may provide a cellular measure of the aggressive state in the CNS. (3) Anatomical tracing, stimulation and lesion experiments will be performed to identify brain regions that provide in puts to the motor nuclei, and regions involved in the long- term modulation of this behavior. These studies will help to piece together the central pathways that regulate the aggressive state, as well as pinpoint where and how experiential factors act in the CNS to influence aggressive behavior. Violence and aggression are urgent problems in our society. while much studies on its social and behavioral aspects have been made, little is know about the neural mechanisms underlying aggression, partly because animal models amenable to detailed cellular physiological studies are rare. The Siamese fighting fish may be a valuable model for exploring the cellular bases of aggression. Only when we understand these bases will we be able to interfere with the aggressive process. The proposed studies aim at seeking fundamental and general rules that may be useful in indicating the directions that aggression research in higher mammals, including humans, should take, the kind of variables that should be considered, and hypotheses that should be tested.